(1) Field of the Invention
This invention relates to radio frequency identification devices and, more particularly, to radio frequency identification devices molded of conductive loaded resin-based materials comprising micron conductive powders, micron conductive fibers, or a combination thereof, homogenized within a base resin when molded. This manufacturing process yields a conductive part or material usable within the EMF or electronic spectrum(s).
(2) Description of the Prior Art
Radio frequency identification devices (RFID) are becoming increasingly popular for a variety of industrial, retail, transportation, and other applications. RFID technology provides a positive identify for any object, person, or the like, bearing the RFID transponder through the use of a passive, radio frequency signal. In a typical technology, a RFID transponder comprises an antenna and an integrated circuit. When a separate, RFID reading device broadcasts a radio frequency signal, this signal interacts with the RFID transponder antenna. The transponder antenna converts part of the received RF signal energy into an electrical current. This electrical current, in turn, is detected by the integrated circuit and, in many applications, actually powers the integrated circuit. The integrated circuit then modulates this electrical current in the transponder antenna to thereby create a return RF signal. This return RF signal is then detected by the antenna in the RFID reading device. This modulated, RF return signal carries encoded data about the transponder based on the data previously stored in the integrated circuit. For example, the serial number of the transponder may be returned to the RFID reading device via this modulated RF signal. Finally, the RFID reading device decodes the signal returned from the transponder to complete the identification.
RFID transponders find a large and growing number of applications. Employee identity badges, animal identity devices, retail pricing and inventory devices, retail security devices, manufacturing product and material tracking devices, vehicle identification devices, and the like, are just a few examples of the growing area of applications for RFID technology. RFID transponders are ideally suited for integration onto a wide variety of products and into a wide variety of situations. RFID transponders may be purely passive devices where all of the energy for operating the integrated circuit is derived from the broadcast RF signal. Alternatively, active RFID systems incorporate an on-board battery to provide power to the identity chip and/or power for the transponder's return RF signal. In fixed systems, such as motor vehicle transponders used for automated toll collection, the additional cost of the on-board battery is easily justified by the improved performance of the device. Conversely, in very cost sensitive applications, such as retail pricing and security tags, the RFID transponder device must be as inexpensive as possible and, therefore, must be a passive system.
The on-board antenna is a key enabling technology for RFID transponder devices. The broadcast RF energy may be in the form of a magnetic field, an electric field, or a mixed field as in typical radio signal broadcast. The transponder antenna is designed with a shape and a size based on the characteristics of the broadcast RF energy such as the field type and the signal frequency. In many applications, this transponder antenna is manufactured as a spiral, or loop, structure to maximize magnetic field interaction and energy transfer. Two general types of antenna material are used: metal and conductive ink. Metal antennas are typically formed either as a three-dimensional coil, as found in plastic in-store anti-theft devices, or as a two-dimensional spiral structure, as found in employee badges. Metal coil antennas are formed by winding metal wire onto a core or frame to create a coil. Coils are large and expensive to manufacture. Flat metal antennas are typically formed from a thin layer of metal, such as copper or aluminum, which is laminated onto an insulating substrate material. This metal layer is chemically etched to form the desired pattern. Further, to provide connectivity between the two ends of a spiral antenna and the identity chip, a second conductive layer must be defined. The two-dimensional spiral antenna provides a much smaller antenna that is well-suited to smaller transponder designs. However, the manufacturing cost is still too high for many applications.
The second conductive layer in a flat metal antenna comprises a conductive ink in some prior art designs. This conductive ink may further be applied to the formation of the spiral antenna itself. Conductive ink may be applied using screen printing techniques. However, conductive ink has disadvantages. First, conductive inks are of higher resistivity than metal and, therefore, result in lossy antennas. In a passive RFID transponder, the system performance is directly related to efficient RF energy reception and conversion. Conductive ink antennas provide reduced system performance due to resistive losses. Second, spiral conductive ink antennas require two conductive ink layers separated by an insulating ink layer to complete the circuit connectivity. The manufacturing process requires multiple steps and tooling and requires that the ink layers maintain layer-to-layer registration. As a result, the manufacturing process can be complex and expensive. It is a key objective of the present invention to provide a RFID antenna structure and a method of manufacture that each improve upon the present art.
Several prior art inventions relate to RFID transponders and methods of manufacture. U.S. patent Publication 2002/163434 A1 to Burke teaches an RFID tag with a metallized dielectric substrate. U.S. Pat. No. 6,078,791 to Tuttle et al teaches a radio frequency identification transceiver and antenna that utilizes a conductive polymer material to connect a crossed antenna structure. U.S. Pat. No. 6,741,178 B1 to Tuttle teaches an electrically powered postage stamp or mailing or shipping label operative with radio frequency communication.
U.S. Pat. No. 6,466,131 B1 to Tuttle et al teaches a radio frequency data communication device with adjustable receiver sensitivity and a method of manufacture. This invention teaches an antenna that is silk screened onto an ID card with a conductive polymer thick film. U.S. patent Publication 2004/0074974 A1 to Senba et al teaches a RFID housing structure made of a conductive metal which has an excellent strength and durability that has a gap or path for the electromagnetic waves to be able to communicate with a read/write terminal. U.S. Pat. No. 6,032,127 to Schkolnick et al teaches an intelligent shopping cart utilizing a radio frequency base station and antenna and is able to communicate with the RFID tags on the items in the cart, total the price of the items, and accept payment as well as communicate with the main terminal in the store.
U.S. patent Publication 2001/0054755 A1 to Kirkham teaches an integrated package and RFID antenna utilizing conductive resins, conductive inks, conductive polymers or metals in the package and capacitively coupling the package to the RFID antenna to create a much larger antenna. U.S. patent Publication 2003/0069793 A1 to Rudolph et al teaches a tax stamp authentication and product control RFID tag for Cigarette packaging that utilizes an antenna comprising a conductive ink, conductive resin, conductive polymer and mixtures thereof. U.S. Pat. No. 6,411,213 B1 to Vega et al teaches a radio frequency identification tag system using tags arranged for coupling to ground that utilizes a back plate formed from conductive plastic to enhance electrical coupling of radio frequency identification tag to the body of the animal or person.